The strength trainability in children has been widely explored, however, there is still a questioning with regard to how strength decreases when they stop training. The objective of this study was to evaluate the effect of 12 weeks of muscular strength detraining of boys trained for 12 weeks. Seven prepubertal boys (EX 9.4 ± 1.6 years of age) trained three series of 15 repetitions, three times a week for 12 weeks. The training, supervised and developed in equipments, consisted of eight exercises including knee extension (KE) and elbow flexion (EF). The 1-RM test of NE and EF was performed before and after training and 12 weeks after detraining. A similar group of boys (n = 7, 9.7 ± 1.7 years), who did not train served as control (CO). After training, the group EX increased (p < 0.05) 1-RM from 14.6 ± 9.8 to 26.2 ± 12.9 kg in KE, and 4.7 ± 2 to 7.9 ± 4.1 kg in FC. After 12 weeks of detraining, the 1-RM was 19.6 ± 11.2 in NE and 6.5 ± 3 kg in FC. The decrease on strength was not statistically significant (p > 0,05). When corrected by the body weight and by the lean body mass (LBM), the 1-RM of NE decreased significantly (p > 0,05) from 0.64 ± 0.1 to 0.45 ± 0.1 and from 0.83 ± 0.2 to 0.61 ± 0.2 of the body weight and LBM, respectively. The EF strength did not decrease significantly when corrected by the body weight and LBM. The strength levels did not change in the first 12 weeks for group CO, however, after 24 weeks, it presented an increase of 41% in the 1-RM of KE and 53% in EF. One concludes that, after detraining, the muscular strength presented no significant reduction in absolute values; the results are significant only when corrected by weight and LBM and it is only evidenced for the lower limbs.